In recent years, studies have been made on electrical wires coated with a halogen-free flame retardant resin composition which is low fuming and generates less harmful gas such as hydrogen halide, from the viewpoint of fire safety. As the halogen-free flame retardant resin composition, those comprising a polyolefin-based resin as a base polymer and a metal hydrate such as aluminum hydroxide or magnesium hydroxide in a large amount as a flame retardant are mainly used.
However, polyolefins used as the base polymer, such as polyethylene, ethylene/α-olefin copolymers, ethylene/vinyl acetate copolymers (EVA) and ethylene/ethyl acrylate copolymers (EEA), are, more or less, crystalline polymers having a crystal part within a normal temperature region, and therefore have relatively high rigidity and require the incorporation of a flame retardant metal hydrate which is an inorganic compound in a large amount, thereby failing to provide sufficient elongation, flexibility and molding processability required of electrical wire molded bodies. Therefore, a low-crystalline, flexible base polymer must be used to reduce the rigidity. However, the reduction in crystallinity of the base polymer causes the problem of reduction in strength of an electrical wire molded body, and also leads to a lowered melting point of the base polymer and a significantly-increased heating deformability even at a relatively low temperature, thereby causing the problem of reduction in heat resistance of an electrical wire molded body. Thus, in order to compensate for such reduction in strength and heat resistance, crosslinking among these polymers is commonly carried out.
While methods such as electron beam crosslinking, crosslinking by means of a crosslinking agent and silane crosslinking are used for such crosslinking, above all, silane crosslinking, advantageously, does not require large-scale production facilities, and is easy to operate. However, silane crosslinking for an electrical wire molded body made of a polyolefin composition comprising a large amount of a metal hydrate as a flame retardant involves the problem of attaining a sufficient degree of crosslinking while preventing early crosslinking by contact between a metal hydroxide and a crosslinking component.
On the one hand, strict flame retardance standards, for example, Vertical Flame Test defined in UL1581 (Reference Standard for Electrical Wires, Cables, and Flexible Cords)), etc., VW-1 standard, horizontal flame retardance standard as well as 60-degree inclined flame retardant characteristic defined in JIS C3005, and the like are defined for electrical wire molded bodies. In recent years, electrical wire molded bodies are required to correspond to more strict flame retardance standards. Besides, from the viewpoint of safety, standards tend to be defined for physical properties other than flame retardance, and there is a growing demand for materials having high flame retardance as well as excellent physical properties including heat resistance.
Therefore, crosslinking steps have been variously reviewed in the production of an electrical wire molded body from a silane crosslinkable flame retardant polyolefin.
For example, Patent Document 1 discloses a process comprising melting and mixing a flame retardant polyolefin comprising a silanol condensation catalyst, a metal hydrate and a water absorbing agent and a carrier polymer comprising an unsaturated silane compound, and then crosslinking the mixture in a moisture content.
Patent Document 2 discloses a process comprising melting and mixing a component comprising a silane grafted polyolefin and a metal hydrate and a component comprising a polyolefin, a silanol condensation catalyst and a crosslinking agent, and then crosslinking the mixture in a moisture content.
Patent Document 3 discloses a process comprising melting and mixing a resin composition comprising a polyolefin, an organic peroxide, a silanol condensation catalyst and a metal hydrate and a silane-modified polyolefin, and then crosslinking the mixture in a moisture content.
However, these crosslinking processes are problematic in uniform dispersibility of the metal hydrate, complicated steps and the like, and no process for efficiently producing an electrical wire molded body having not only flame retardance but also other properties at high levels has been obtained.